With a recent development of semiconductor techniques, there is a demand for a high-purity silica sol containing very few metal impurities for the silicon wafer polishing and in the CMP process of semiconductor devices because it does not contaminate materials such as silicon wafers.
The production of the silica sol containing silica at a high concentration, i.e., a highly-concentrated silica sol, is preferred because it not only raises the production efficiency but also increases the storage/transport efficiency. Further, it is preferred because it freely enables to control the compounding of polishing agents, i.e., it increases compounding flexibility. Thus, various reports regarding the highly-concentrated silica sol have been made.
Regarding the use of a silica sol in the silicon wafer polishing, it is a problem that a recycled silica sol shows a viscosity-increase which interferes with its efficacy as a polishing agent. Accordingly, a silica sol showing long-term stability and low-viscosity, even after recycling, is desired.
A silica sol with small silica particles is particularly used for the final polishing of 300 mm wafers, and it increases the flatness without causing damage (scratch-free). This silica sol is also used as a polishing agent in the CMP process for LSI provided with remarkably-miniaturized elements. The silica sol with small silica particles has high precision capable of polishing materials required to be scratch-free flat and microstructural. Thus, the highly-concentrated silica sol with small silica particles is needed.
For example, Patent application Tokukoushou No. 37-9961 (publication) discloses a method to prepare a 30% or higher concentrated-silica sol by adding monovalent cation soluble metal salts (alkali metal salt) as a dispersing agent. This method produces the highly-concentrated silica sol while maintaining a low viscosity. According to this method, the silica sol contains alkali metal salt-derived metal impurities so that a high-purity silica sol cannot be produced.
Another method using ammonium salts containing lower alkyl as a dispersing agent is also disclosed in the same document, which produces a silica sol containing no metal impurities. However, the ammonium salt is degraded in a heating process because its degradation temperature is low. This results in inadequate efficacy of ammonium salt as a dispersing agent. Moreover, the viscosity of the silica sol cannot be maintained at a constant level because the dispersing agent is degraded or vaporized during long-term storage. Thus, the viscosity is increased and the gelation is induced with time.
To solve the above problems, a silica sol containing metals at a very low concentration has recently been produced. However, the silica concentration is about 15 weight %. A production of the 15 weight % or lower silica sol has problems such as low production efficiency and low compounding flexibility of the polishing agent. It is also inappropriate with regard to transport and storage.
There is another problem regarding the production of a silica sol with small silica particles. The silica sol with small silica particles is likely to aggregate and induce the gelation of solution, compared to a silica sol with large silica particles. In consequence, the viscosity easily increases, which makes the production of the highly-concentrated silica sol with small silica particles difficult.
The silica sol with a high concentration, low viscosity and high purity in spite of any silica particle size differences is strongly desired, but no silica sol satisfying these conditions has been produced.
In view of the above problems, this invention aims at providing a high-purity and highly-concentrated silica sol with long-term stability and low viscosity by preventing the viscosity-increase during storage after the production, and methods for producing the same. This invention also aims at providing a high-purity and highly-concentrated silica sol which does not undergo the viscosity-increase or the gelation in spite of any silica particle size differences in a silica sol by adding a dispersing agent at a specific concentration to a reaction or a solvent-substitution-concentrated-solution and production methods thereof.